Yeah that don't look good. You can see the 2nd picture where there's a thin line of the glass cracked that doesn't terminate yet so there's still more cracking to be done. Now if that was a superficial chunk of glass taken out of the edge I would be less worried, The silicone looks fine, and for a smaller sized tank I wouldn't be too worried, but yeah that crack doesn't look finished and there's no telling which direction it could go.
Was that black backing covering that up at all this whole time? The seller maybe didn't know? Or worse... did know.